Paint can "flash" or leave streaks for several reasons, some are controllable and some not. Plus some or possibly all of the streaking can disappear after the paint has cured, typically 2 weeks.
1) Using a low grade of house paint can cause streaking. Always use the best paint you can afford. 100% acrylic house paint is the best and will out perform latex paint.
2) The paint is drying to fast. Water based paint can be conditioned by adding Floetrol Paint Conditioner. This will allow the paint film to level out on the wall and extend its wet edge time.
3) Excessive rolling can cause paint flashing. It's best to apply interior house paint quickly and evenly with out excessive rolling.
4) Dark paint colors can show streaks. This depends on the color and paint manufacture. Using Floetrol can help, but some streaks might be inevitable.